Planarity is a simple game, but difficult to master. You're given an arrangement of dots in a circle. Each dot is connected to 2, 3 or 4 other dots via lines, and as you move the dots, the lines stay attached to the dots. The object is to arrange the dots so that none of the lines cross. Try it for yourself to see how challenging this can be.

Once you've tried it for yourself, play the video above, in which TimwiTerby not only solves a level 18 puzzle, but does so by arranging them in perfect rows!

If that isn't impressive enough, check out the level 19 video, in which the final arrangement is a series of concentric circles!

It's not completely done in your head, but since you can write down the digits as you go, it can appear that way. It looks especially good if you can remember the numbers you carry, instead of writing them down.

While I post videos of amazing feats here with descriptions, I also like to maintain a full index of the original links on their respective sites. They are listed in pulldown menus below, divided up by format. Each individual video is listed alphabetically by the performer's last name, screenname or title.

Originally, these links were hosted in the sidebar, but it has become too large and unwieldy to update this on almost every page of the Grey Matters site.

Instead of the index itself, you will now see "Full Video Index" in the sidebar, which will be linked to this post. Also, as new videos are added, this post will be updated.

In the numbers round of this March 1997 episode, the contestants are given the random numbers 25, 50, 75, 100, 3 and 6, and must use only those numbers in a formula to get as close as possible to the randomly-generated total of 952. Contestant James Martin not only gets a total of 952, but does so in an amazing way!

If you haven't already watched the video, try and figure out for yourself how close you can get to 952.

James Martin has to be one of the most memorable game show contestants since Michael Larson.